Re: [Haskell-cafe] Let's teach GHC idiom brackets!

FWIW, the original paper, http://strictlypositive.org/Idiom.pdf, defines a neat trick to encode idiom brackets with no need for any extensions as a solution to exercise 3 in the paper.

Here's the trick explicitly for anyone who was wondering (like me) https://gist.github.com/mpickering/99d62ccdb73f49840220 On Thu, Feb 19, 2015 at 10:37 AM, Mathieu Boespflug wrote:

Hi Ollie, Chris,

nice idea. This is something I've always thought would be nice to have since the original paper on applicatives. However, Tikhon, Ed and Henrik raise an important point: how would I write the following using idiom brackets?

(,) <$> g x y <*> h (g x y)

or

f <*> g x y <*> h ((,) <$> g x y)

Does g x y need to be written (| g x y |) for the Identity Applicative?

FWIW, the original paper, http://strictlypositive.org/Idiom.pdf, defines a neat trick to encode idiom brackets with no need for any extensions as a solution to exercise 3 in the paper.

Best,

Mathieu

On 19 February 2015 at 10:56, Oliver Charles wrote:

...

Christopher Done writes:

Hey Chris, thanks for chiming in!

...

Here's a fair comparison of before/after on my ace package's parser: https://github.com/chrisdone/ace/commit/36767780ca356db75468f803b0c91388186c...

In my opinion, this is exactly why I want this extension - these changes are easier for me to parse after the changes, and would be even nicer if we could drop the 'i' (so the brackets are symmetric).

Interestingly, I think some of these changes could be even nicer assuming we had ApplicativeDo as well:

(|SentenceCoord_2 sentenceCoord_3 (optional (try (do string "or" sentenceCoord_2)))|)

As you can see, it allows us to do away with infix operators entirely.

...

Cons:

* Nesting is not supported by quasi-quoters.

This is something I think is very important - I often nest applicative syntax in my own code, and it's essential to me that I would be able to nest idiom brackets too.

...

* Currently it lacks the Alternative support as proposed by Conor. That would look like this:

[i|TopicalizedSentenceExistential existentialTopic (optional (try sentenceCoord)) |TopicalizedSentenceUniversal universalTopic sentenceCoord |TopicalizedSentenceComposite compositeSentence|]

I'm actually somewhat against this - my personal preference is that we keep this extension very minimal. Essentially, all we're doing is overloading what whitespace means within the idiom brackets. Introducing new symbols now complicates that, and introduces yet more syntax.

My current solution for dealing with sums is to use asum:

asum [[i|TopicalizedSentenceExistential existentialTopic (optional (try sentenceCoord)) ,[i|TopicalizedSentenceUniversal universalTopic sentenceCoord |] ,[i|TopicalizedSentenceComposite compositeSentence|]]

Though it is a little noisy, I think it gives you want you want - machine-predictable indentation and consistency.

...

Comparatively, I've never used arrows ever and yet GHC has built-in support for it. Meanwhile Applicatives have wide application (hehe) and we're stuck writing this clunky stuff. But I figured I'd play with the quasi-quoter and see how it feels, and see if other people get onboard with it.

Interestingly there is more interaction there. It's somewhat widely known now that Arrow <=> Applicative + Category. Thus in my current codebase using Opaleye, I actually work entirely *without* arrow notation. I find the notation alien-enough that it's not providing a huge win for my productivity, but in return I end up paying a cost in a little more typing.

personBirthdayProduct = proc () -> do personRow <- personQuery -< () birthdayRow <- birthdayQuery -< () returnA -< (personRow, birthdayRow)

right now becomes

(,) <$> personQuery <*> birthdayQuery or liftA2 (,) personQuery birthdayQuery

but with idiom brackets becomes

(| (personQuery, birthdayRow) |)

A slightly more complicated example is

youngPeople = proc () -> do row@(_, age, _) <- personQuery -< () restrict -< age .<= 18 returnA -< row

could become

(| fst (second (lmap age restrict) . (| dup personQuery |)) |) where dup a = (a,a)

Arguably not considerably better as you have to tuple things up only to immediately unpack them, but hopefully provides some insight how the use of ArrowNotation could be replaced with idiom brackets.

- Ollie _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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